Lift trucks are well-known vehicles for handling loads. Conventional lift trucks include a pair of forwardly projecting forks for engaging the underside of the load. The forks are mounted to a carriage that is driven for lifting and lowering the load that is carried by the forks.
Lift trucks are commonly used in warehouses, transfer yards, etc., for handling a variety of items. A large percentage of the items are loaded into containers or on pallets. The lift truck forks are maneuvered by the lift truck operator into receiving pockets provided in the containers or pallets. Controls enable the operator to raise the forks and its load which is then transferred as desired. (Hereafter references to pallets will be understood to be interchangeable with containers otherwise used for carrying or containing items which make up a load).
Several attachments are available for enhancing the capabilities of a lift truck. One important attachment is known as a side shifter. A side shifter attaches to the lift truck carriage and provides a mechanism for lateral movement of the forks. With side shifting capability, a lift truck operator does not have to precisely align the lift truck with the pallet pockets. If the forks are not aligned in the desired position, the side shifter can be engaged to position the forks in the desired position. Side shifters eliminate a substantial amount of lift truck movement that would otherwise be required to achieve the lateral movement of loads or forks.
A lift truck carriage includes upper and lower horizontal crossbars. A side shifter generally comprises a rigid frame that attaches to those crossbars. In this regard, the side shifter frame includes upper and lower cross members. The upper cross member is mounted to the upper crossbar of the carriage, and the lower cross member is secured by one or more retainers to the lower crossbar of the carriage. The lift truck forks are then mounted to the side shifter frame.
A drive mechanism, such as a hydraulic cylinder, is interconnected between the lift truck carriage and the side shifter frame for moving the side shifter relative to the carriage. The side shifter retainers are configured to secure the side shifter frame to the carriage while allowing the side shifter to slide along the carriage. Low-friction slide blocks are placed between the side shifter and the carriage to facilitate the sliding movement of the side shifter.
A problem with prior art side shifters is that the rigid frame for the side shifter is bulky and reduces forward vision for the operator of the fork lift truck. This makes it difficult to maneuver in tight places and to precisely position loads with the fork lift truck.
The upper and lower crossbars of the lift truck are structural pieces that must be capable of supporting and handling heavy loads. The finish on the crossbars is usually rough and becomes rougher during use. The lower crossbar usually has an "as rolled" surface finish that contains mill scale. The "as rolled" surface has a high coefficient of friction and provides a very poor surface on which to slide a side shifter. The high friction surface on the crossbars of the fork lift truck increases the force necessary to slide the side shifter and increases the wear on the components of the side shifter that slide over the crossbars. To further compound this problem, contaminants frequently are present in the interface between the crossbars and the side shifter. This is especially true with regard to the lower crossbar. The contaminants can further reduce the slideability of the side shifter on the crossbars.
Most modern lift trucks are built in conformance with standards recommended by the Industrial Truck Association (ITA) or by the International Organization for Standardization (ISO). One such standard establishes the minimum clearance that should exist between the ground and the underside of the lower crossbar of a lift truck carriage whenever the carriage is in its lowest position. In the past, the retainers used to secure side shifters to the lower crossbar of the carriage were so configured than they extended substantially below the underside of the crossbar. As a result, the clearance between the ground and the lowermost portion of the side shifter (i.e., the underside of the retainer) was often significantly less than the recommended standard referred to above. Consequently, prior retainers occasionally contact raised obstructions on the surface over which the lift truck operates. Such contact may damage the side shifter or cause dangerous shifting of the load.